1. Field of the Invention
The present invention relates to a semiconductor power module, and particularly to improvement for suppressing a surge voltage.
2. Description of the Prior Art
A semiconductor power module is a device which comprises a circuit which controls electric power by using an active semiconductor element for electric power control. Some semiconductor power modules further comprise a control circuit which comprises an active semiconductor element. In a semiconductor power module comprising a control circuit, the circuit for controlling electric power mentioned above is regarded as a main circuit and the active semiconductor element of the control circuit controls operations of the main circuit by means of signal transmission between the active semiconductor element and the main circuit. The most commonly encountered application of a semiconductor power module is an invertor or the like for controlling operations of a motor or the like.
FIG. 14 is a plan view of a circuit part of a conventional semiconductor power module. In this device, the normal rated output power is about 0.5 kW and an electric power is cyclically blocked and connected at a frequency of about 5 kHz. In the circuit of this device, conductive wiring patterns P(P), P(N), P(U), P(V), P(W) and P(G1) to P(G6) are formed on the top surfaces of insulative circuit board bodies SB1 to SB3. Insulative gate bipolar transistor elements (IGBT elements) Ta1 to Ta3 and Tb1 to Tb3, which are each constructed as an electric power control semiconductor element, are disposed on the top surfaces of the conductive wiring patterns P(P). IGBT elements Ta4 and Tb4 are disposed on the top surface of the wiring pattern P(U), IGBT elements Ta5 and Tb5 are disposed on the top surface of the wiring pattern P(V), and IGBT elements Ta6 and Tb6 are disposed on the top surface of the wiring pattern P(W). The conductive wiring patterns P(P), which are respectively formed on the top surfaces of the circuit board bodies SB1 to SB3, are electrically connected to each other by jumpers J1 and J2 while the conductive wiring patterns P(N) are electrically connected to each other by jumpers J3 and J4 in a similar manner. The portions shadowed by oblique lines in FIG. 14 represent terminals which are connected to the wiring patterns. A number of conductive wires w attain electrical connection between the IGBT elements and the wiring patterns and electrical connection between the wiring patterns.
The wiring patterns P(P) and P(N) transfer a positive and a negative power source potentials, respectively, and supply a power source current to the IGBT elements. The wiring patterns P(U), P(V) and P(W) each carry each one of three-phase output currents. The wiring patterns P(G1) to P(G6) are wiring patterns for transmitting gate voltages which are developed at the IGBT elements. By connecting an external power source (not shown) to power source terminals PS(P) and PS(N) to which the wiring patterns P(P) and P(N) are connected, the power source potentials and the power source current are supplied to the IGBT elements.
The wiring patterns P(P) and P(N) are located near opposite ends of the circuit board bodies SB1 to SB3. Between the wiring patterns P(P) and P(N), other wiring patterns including the wiring pattern P(U), the IGBT and other elements are disposed. Mounted near the opposite ends of the circuit board bodies SB1 to SB3, the power source terminals PS(P) and PS(N) are spaced apart from each other.
A semiconductor power module having a high frequency is desired since the higher frequency for cyclically blocking and connecting electric power a semiconductor power module has, e.g., about 10 kHz or higher, the better performance the semiconductor power module attains including reduced electric power loss, improvement in response and operation accuracy of an object to be electric-power controlled such as a motor or etc. Another demand for a semiconductor power module is the ability of controlling a larger electric power, for example, an electric power of around 1 kW or more, which is necessary to drive a large, industrial use motor or the like.
The power source current mentioned above intermittently flows because of operations of the IGBT elements. As the power source current flows intermittently, due to a parasitic inductance which is present in the path of the power source current which extends from the power source terminal PS(P) to the power source terminal PS(N) through the wiring patterns P(P), the IGBT elements and the wiring patterns P(N), a surge voltage is developed in the path. The surge voltage increases in proportion to an increase in the value or the frequency of the power source current which is cyclically blocked and connected. If excessively high, the surge voltage causes an electrical noise which leads to circuit failure in the device and eventually to destruction of the circuit elements which form the circuit.
In the conventional device as above, a rather large parasitic inductance exists in the path of the power source current. Hence, circuit failure or destruction of the circuit because of the surge voltage would not be possibly prevented by mere modification in the circuit design such as an increased current capacity of the wirings of the circuit board by incorporating high speed, large current capacity semiconductor elements for electric power control in the structure of the conventional device. Clearly, this is not the answer to the demand for a large power, high frequency semiconductor power module.